Double walled insulated tubing and method of installing same

ABSTRACT

A double walled insulated tubing string (46) adapted to be hung from a well head (58) has an inner tubing string (21) formed of plural inner tubes and a radially outer tubing string (22) formed of plural outer tubes, the strings being independent of one another apart from axially fixing the strings relative to one another at the top end of the tubing string, although axial fixation may also be provided at the lower end of the tubular strings if desired. An annular insulating gap (30) between the tubing strings (21, 22) is hermetically sealed at both opposing ends and may be evacuated. Inner and outer tubing strings may be run into a well using conventional oil well drilling technology and there is no mechanical connection between the inner and outer tubing strings (21, 22), especially at connections of the respective inner and outer lengths of tube. To maintain the inside of the outer tubing string (22) free from formation liquids before the inner tubing string (21) is run into the well and sealed against the outer string at its lower end, a temporary plug (31) initially closes the lower end of the outer string and is de-activated before liquid can flow through the thermally insulated dual tubing string (46).

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to double walled insulated tubing and to a methodof installing such tubing in a well, such as a geothermal or oil well.

There is requirement to provide excellent thermal insulation propertiesin a tubular pipe string where it is required to bring warm or hotliquids to the surface from a deep depth. In other words, it is requiredto keep the liquid cooling on its traverse from the bottom of the wellto the top of the well to a minimum, even though the temperature of thesurrounding formations may drop by an average of3° C. per 100 meters.Such applications are as follows:

1. Geothermal wells which produce hot water from deep aquifers at ratherlow flow rates where the thermal water would otherwise cool aconsiderable amount on the traverse to the surface, thereby losingthermal energy available to surface consumers.

2. Closed loop geothermal wells where thermal heat exchanger liquid,which is commonly water, is pumped down to the bottom of a well in anannular space formed between a cemented casing string and a tubingstring while drawing thermal energy from the surrounding formations andsubsequently transporting the thermal energy to the surface through thetubing string.

3. Oil wells producing crude oil with a high bitumen or paraffincontent. Without an efficient thermal insulation, the oil in the tubingstring cools considerably as the oil flows towards the surfaceinstallation. Once the temperature drops below a level which is specificto the type of oil, bitumen or paraffin starts to change to the solidstate and adhere to the internal wall of the tubing string. As a result,flow resistance increases due to the decreased cross-section of thetubing so that sucker rods may become stuck and break. To pump cool and,therefore, highly viscous oil through the pipeline sections close to thewell head requires an unnecessary high amount of energy.

4. To produce oil that is already highly viscous in the downhole oilbearing formation where hot steam is pressed into the formation aroundthe well bore to heat up the viscous oil and, thus, improve its flowbehaviour. Heat losses on the way to the surface are required to be lowso as not to cause any unnecessary flow pressure drop in the productiontubing string.

(2) Description of the Related Art

Pipe strings that are covered with glass or stone wool and then wrappedwith foils or strips of thin steel sheets to shield the insulating layeragainst water are frequently used for surface applications. However,water may enter the insulating material through small holes or cracksthat may occur in the surrounding cover and reduce the insulatingproperties of such a string. Therefore, this known method cannot be usedto thermally insulate production tubing strings exposed to high pressureliquids in the annular space between the tubing string and a deep wells'casing.

Pipe made from fibreglass has a lower thermal conductivity than steeland is usually used for pipelines or tubing strings for corrosive mediarather than for the purpose of thermal insulation because the thermalproperties are usually insufficient. Temperature limits and lowerstrengths further reduce the possible range of applications of pipe madefrom such material. Fibreglass material is also considerably moreexpensive than steel pipe of the same diameter and cannot be used in oilwells equipped with reciprocating sucker rods. Also, inventory costswould rise if fibreglass tubing has to be kept in stock in addition tothe standard steel tubing that is generally used in oil fieldoperations.

Hitherto, thermally insulated injection and production tubing stringshave sometimes been used to complete steam injection wells to obtainincreased thermal efficiency of the system. The strings, which oftenhave a length exceeding 1,000 meters, are constructed from individuallengths of double wall pipe, each of which usually has a length of about9 meters--that is equivalent to range 2 API tubing joints--to be run andretrieved by oil field rigs in the same way as single wall strings.

The most common version of such a string is shown in the CompositeCatalogue of Oil Field Equipment, Volume 1, page 988H, 35th revision,1982-83, published by Gulf Publishing in the U.S.A. The prior art tubingstring will now be described with reference to FIGS. 1 and 2 of theaccompanying drawings in which FIG. 1 shows a longitudinal cross-sectionof a tubing string and FIG. 2 shows an enlarged, detailed, longitudinalcross-sectional view of a part of the string shown in FIG. 1 to moreclearly show the connection between two lengths of tubing.

The thermally insulated tubing shown in FIG. 1 depicts one individuallength of thermally insulated tubing having connections at opposing endsthereof, but it is to be realised that in practice there would be aplurality of such tubes lengths used to form a thermally insulatedtubing string. FIG. 2 shows constructional details of the threadedconnections used to form a length of insulated tubing connected one tothe other.

In FIGS. 1 and 2, an internal tube 1 has the internal diameter thereofincreased at opposing ends and the internal tube 1 is positionedconcentrically inside an external tube and the opposing ends of theinternal tube are welded to the external tube 2. Located in the annularspace 3 formed between the external wall of the internal tube and theinternal wall of the external tube is an insulation medium such as air.The external tube 2 is slightly longer than the insulated double wallsection so that connecting screw threads can be cut at opposing ends ofthe external tube. Double box couplings 4 are used to link one length ofdual insulated tubing to the next dual tube in an axial direction. So asto avoid internal diameter changes at each joint connection in thestring, which would undesirably increase dynamic pressure losses in theflow of fluid, e.g. oil, non-insulated section sleeves 5 are inserted atthe tube ends at the time of running the string into the well.

Mechanically, such strings fulfil all the strength requirements for deepwells. However, even in the ideal case where liquid from the well boredoes not enter into the annular space 6 between the sleeve 5 and the boxcoupling 4, a considerable amount of thermal energy is lost through thethermally conducting bridges created by the internal and external tubesbeing welded together and also by there being a mechanical connectionbetween the sleeve 5 and the box coupling 4. However, water or oil willusually enter the annular space 6 to further reduce the efficiency ofthe insulation. Any mechanical damage to the internal tube 1 caused byreciprocating sucker rods or by corrosion will permit liquid to flowinto the annular space 7 between inner and outer tubes, thereby causinga thermal bridge that will not immediately become visible at the surfacesince the annular space is closed off at the end of each internal tubeby the internal tube being welded to the external tube.

Another short coming of this type of insulated string is the requirementto weld the internal and external tubes together. In the drillingindustry, it is usual to avoid, wherever possible, welding tools orequipment that is positioned downhole because such welds are thestarting points for corrosive leaks. Thus, in the present thermalinsulated tubing string example, welding between the internal andexternal tubes may well be the starting point for a corrosive leak. Ifcorrosion starts from inside the annular space 3 between the two tubes,it will neither be detected visually nor by non-destructive inspectionmethods commonly used in the oil and gas industry.

When steel tubes leave the production line of a factory they normallyhave wide length tolerances. So as to match internal and external tubesof dual wall tubing, tubes have to be cut to matching length, thusincreasing material cost. The main reason why dual wall tubing stringsare not more frequently used is the high cost of purchase, being amultiple of the cost of plain tubes, as well as the increased deliverytimes for the especially manufactured welded joints and, lastly, but notleast, increased inventory levels.

Another dual wall pipe designed for thermal insulating purposes isdisclosed in EP-A-0138603. This reference overcomes the difficulty ofthe above-mentioned prior art by providing a passageway between theconnection of two connected lengths of tubing so that the annular spacein the respective lengths of tubing are connected together. Thus, inEP-A-0138603 the ends of the internal and external tubes are connectedtogether by a wall and a small passage having a smaller cross-sectionalarea than the cross-sectional area of the annular space extendslongitudinally axially from the annular space outwardly of theconjunction of the internal and external tubes. Because the location ofthe passage in each tube may not exactly align with one another, so thelongitudinally axially outermost portion of the passage is enlarged incross-sectional area so as to ensure that when two lengths of dualwalled pipes are connected together, their respective passages will beinterconnected. At an internal and external side of the passage wherethe two lengths of tubing are connected together there is providedrespective annular seals to create a pressure seal between the annularspace inside the insulated tubing string and both the liquid inside andaround the insulated string.

The annular space may be filled with any desirable insulated gas orliquid and may, alternatively, be evacuated from the surface. Thus, theinterconnected annular space can be used to check for leaks in any ofthe seals or in the walls of the tubing. If the annular space isinitially filled with gas, a leak is indicated by an increase inpressure which will rise until the pressure in the annular spacebalances the pressure of the liquid, either inside the insulated stringor outside of it. Once again, this reference has the disadvantage of theabove-described insulated dual wall tubing and if either the internal orexternal tubing should have a leak, the leaking substance will extendthroughout the annular space of the whole string. In such an event, itis difficult to locate the position of the leak which may be inelastomer seals radially inward and outward from the passage.

Another known dual wall pipe for reverse circulation drilling isdisclosed in GB-A-1204026. In this reference, two concentric tubes areconnected to one another by fins that are welded in the annular spacebetween the internal and external tubes. The internal tube is recessedinto each end of the external tube and the external tube is providedwith a screw thread for connecting individual lengths of tube together.When a string is run into a well, a bridging sleeve having seals atopposing ends thereof is inserted over the internal tube of the upperand lower string section to seal the internal tubes of two differentdual wall sections together. Thus, the sleeve extends into the nexttubing joint where it is sealed against the internal tube of the nextsection. The requirement for sleeves connecting the internal tubes toone another adds to the cost of purchasing, storing and maintaining thestring. However, a dual wall tubing constructed in accordance with thisreference also has the disadvantages mentioned above in that theinternal and external tubes are mechanically connected together, therebycreating a thermal connection between the inner and outer tubes.

A thermally insulated pipeline for transporting liquids and gas over theEarth's surface is disclosed in WO91/19129. This reference discloses twoconcentric steel tubes spaced by an annular space and in the annularspace is an insulating material formed of micro-glass fibres ormicro-mineral fibres having a compressive strength sufficient to keepthe inner tube suitably spaced from the outer tube. The insulatingmaterial in the annular space is required to carry the weight of theinternal string, since the tubular strings are approximately horizontal,without the insulating material losing its insulating properties. So asto have the appropriate insulating properties, solid insulators arereferred to which are extremely porous. These support insulators add tothe cost of the tubing string but, even worse, liquids which may passinto the annular space through a leak in one of the internal or externaltubular strings will enter the pores of the insulating material of theinsulators so that the material has to be replaced.

There is no disclosure in this reference of running such a string ofgenerally independent concentric tubular members designed for more orless horizontal pipelines into a liquid filled, generally vertical,borehole or well and where the annular space between the internal andexternal tubes would have to be sealed against the influx of liquidscontained in the well.

Oil field equipment is not designed to simultaneously run or pullconcentric strings of pipe with differing diameters. Oil field tubingjoints, according to the worldwide accepted API standard, do not haveuniform lengths and joint lengths fluctuate considerably. Only withnon-standard, more expensive tubes machined to identical lengths, canconcentric strings of pipe be run simultaneously in a slow operation.Although above the surface pipelines are usually welded because at anytime access can be obtained from the surface at any point along theline, in distinction, downhole tubing strings are usually threadedsince, otherwise, the string has to be cut into slices every time it hasto be pulled from the well for any reason.

SUMMARY OF THE INVENTION

It is an object of this invention to substantially mitigate thedifficulties of the above-mentioned prior art.

According to a first aspect of this invention there is provided a doublewalled insulated tubing string adapted to be hung from a support means,said string comprising inner and outer tubes with a thermally insulatinggap between said inner and outer tubes, characterised in that saidtubing string comprises plural lengths of outer tubes mechanicallyconnected together to form an outer tubing string and plural lengths ofinner tubes mechanically joined together to form an inner tubing string,said inner and outer tubing strings being discrete and separate from oneanother over substantially the whole lengths thereof.

In general, the inner and outer tubing strings are separated from oneanother without any mechanical connection therebetween except at one orboth extreme ends thereof.

In a currently preferred embodiment a removable sealing means isprovided at the lower end of the outer tubing string to prevent liquidingress into the inner tubing string.

Advantageously, the insulating gap is filled with a gas or liquid mediumor said gap is substantially evacuated.

Advantageously, the inner and outer tubing strings are mechanicallyconnected together at the upper end of the tubing string or at bothextreme ends of the tubing string, that is the top and bottom ends ofthe insulated tubing string only.

Advantageously, the outer and inner tubes are adapted to withstand apressure exceeding a hydro-static head of fluid in or outside theinsulated tubing string.

In one preferred embodiment, the inner tube string is pre-tensioned andthe outer tube string is pre-compressed so as not to exceedpredetermined permissible stress levels in the inner and outer tubestrings respectively caused by internal and external temperaturevariations. Advantageously, the inner tube string and/or the outerstring is provided with axial length temperature compensating means.

In another preferred embodiment, an insulating spacing means is providedin said insulating gap for maintaining concentricity between the innerand outer tubing strings so as to avoid the inner tube string contactingthe outer tube string.

According to a second aspect of this invention there is provided amethod of installing a double walled insulating tubing string includingthe steps of:

providing a first length of outer tube with sealing means at a lower, inuse, end thereof for preventing liquid ingress into said outer tube,

mechanically connecting a second length of outer tube to the end of saidfirst outer length remote from said sealing means to form an outertubing string,

suspending said outer tubing string from support means,

locating first and second connected lengths of inner tube forming aninner tubing string inside said outer tubing string, said first andsecond inner lengths of tube being mechanically connected together, andsaid inner tubing string being spaced from said outer tubing string toprovide a thermal insulation gap between the inner and outer tubingstrings, said inner and outer tubing strings being discrete and separatefrom one another over substantially the whole lengths thereof.

Preferably, more than two lengths of outer tube and inner tube areconnected, respectively, together.

In a preferred embodiment, said sealing means is removed by increasingthe fluid pressure within the inner tube lengths or by mechanical means.

Advantageously, to equalise the hydro-static pressure against theoutside of said sealing means in a well bore, the internal tube stringis filled with a liquid and said sealing means is subsequentlyde-activated, for example pumped open.

Advantageously, the inner and outer tubes are mechanically connectedtogether at the upper end of the string or at the extreme ends of thestring, that is at the top and bottom ends of the insulated string only.

Advantageously, the connection between respective outer tubes andbetween respective inner tubes is a leak-proof connection, preferablyprovided by a screw-threaded connection or by welding.

Advantageously, at the lower end of the string a bottom sub is connectedon the outer tubing string and a stinger is connected on the innertubing string so as to form a seal with the outer tubing string.

Advantageously, a seal is provided for the gap at the top of the stringso as to provide a vacuum in said gap.

In a preferred embodiment, where the double walled insulating tubingstring is located in a production casing string, the cross-sectionalarea between said double walled insulating tubing string and theproduction casing string is greater than the cross-sectional area of theinside of the inner tube.

Advantageously, axial length temperature compensating means are includedin either the inner and/or outer tubing string.

An advantage of this invention is that the temperature at the gap at thelower end of the string is approximately equal to the temperature at thelower end of the inner tubing string when in use.

The present invention provides an insulated tubing string having minimumheat loss and which uses normal oil field tubing joints with positiveseal connections without introducing a third, generally tubular,insulating layer of solid material in the gap between the inner andouter tubes, as is required in WO91/19129. If liquid should enter theinsulating gap between inner and outer tubes, it is easy to monitor whena leak occurs so as to locate and replace the leaking element. Nowelding is necessary in the string, although such may be provided ifdesired. Once pulled out of the well, tubing joints are easy to cleanand to inspect so that they can be re-used in the same well or for anyother purpose requiring a tubing string made up from tubing joints ofthe grade of steel used with positively sealing connections.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 shows a longitudinal axial cross-section of insulated double walltubing known in the art,

FIG. 2 shows a detail of a joint used in the known art of FIG. 1,

FIG. 3 shows a longitudinal axial cross-section of a first embodiment ofa double walled insulated tubing string in accordance with thisinvention,

FIG. 4 shows a longitudinal axial cross-section of a second embodimentof a double walled insulated tubing string in accordance with thisinvention,

FIG. 5(a) shows a partial longitudinal cross-section of a tubing stringin accordance with this invention with the inner tube being eccentricwith respect to the outer tube,

FIG. 5(b) shows a cross-section along double arrow-headed lines B--B ofFIG. 5(a),

FIG. 6(a) shows a longitudinal cross-sectional view of a portion of adual double walled insulated tubing string in accordance with thisinvention being centred by centring elements of low thermalconductivity,

FIG. 6(b) shows a cross-sectional along double arrow-headed lines ofB--B of FIG. 6(a),

FIG. 7(a) shows a portion of a double walled insulated tubing string inaccordance with this invention in which the inner tubing string isprovided with thermal expansion elements,

FIG. 7(b) shows a cross-section along double arrow-headed lines B--B ofFIG. 7(a),

FIG. 8 shows a double walled insulated tubing string in accordance withthis invention located in a geothermal well with closed loop heatproduction,

FIG. 9 shows a double walled insulated tubing string in accordance withthis invention in an oil or thermal water-producing well,

FIG. 10 shows a well head for supporting the inner and outer tubestrings and for sealing both strings against one another and alsoshowing provision for attaching a vacuum pump to improve thermalinsulation, and

FIG. 11 shows a lower seal between inner and outer strings.

In the Figures like reference numerals denote like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a longitudinal cross-section of a double walled insulatedtubing string in which there is provided an inner string 21 formed ofplural lengths of inner tube (pipe) joined one to another in the axialdirection and an outer string 22 formed of individual lengths of outertube (pipe) which are axially joined together, the inner string 21being, preferably, concentrically located inside the outer string 22.The inner tube string individual tubes are connected together bythreaded connections 23 and the individual tubes of the outer tubestring are connected together by threaded connections 24 and both thesethreaded connections 23, 24 are, typically, premium type connectionswhich are gas and liquid leak-proof.

At the upper end of the double walled insulated tubing string, the innerand outer strings are secured against relative axial movement withregard one another by a member 25 locking the upper end of the two tubestrings together. Also at the upper end of the strings is a peripheralseal 26 which seals the inner and outer tube strings together at theextreme upper end of the strings. The axial motion prevention member 25and the seal 26 may be combined in a single construction element. At thelower end of the tubing strings, the inner and outer tube strings arealso sealed together by a peripheral seal 27 to close off andhermetically seal a gap 30 between the inner and outer tube strings. Thegap 30 which is, typically, an annular gap since the tube strings areusually circular in cross-section, may be filled with any desiredinsulating liquid or gas via a valve 29 at the upper end of the outerstring 22. Alternatively, the annular gap may be substantiallyevacuated.

A temporary sealing plug 31 is located in the lower end of the outerstring to close off the lower end of the outer string 22 and is sealedto the lower outer tube by a peripheral seal 28. The use of thetemporary sealing plug 31 will be described later herein.

A further embodiment of the invention shown in FIG. 4 which is similarto the embodiment of FIG. 3, except that to avoid excessive relativemovement in the lower seals 27, 28 caused by frequent temperaturefluctuations or caused by friction between the inner string and suckerrods, both independent inner and outer strings are also axially lockedagainst one another by locking member 32 positioned between the seals 27and 28.

Thus, in general, in this invention the inner and outer tubing strings21, 22 are discrete and separate from one another without any mechanicalconnection therebetween over substantially the whole lengths thereofexcept at one or both extreme ends thereof.

Because the wells that tubing strings are located in are rarelyabsolutely vertical or straight, so the longitudinal axis of the innertubing string 21 may not be concentric with the longitudinal axis of theouter string 22. Depending on whole inclination, as well as changes inazimuth and inclination, both strings may touch, as shown in FIGS. 5(a)and 5(b). The eccentricity is, however, limited by the minimum stand-offcaused by the larger outer diameter of the threaded coupling connection23 of the inner tubing string 21 versus the body of the inner tube. Thiseffect may lead to a line contact between inner and outer tubing stringsat some of the couplings causing some minor heat losses. Such heatlosses, however, are significantly less than those associated with theprior art.

FIGS. 6(a) and 6(b) show how these minor heat losses may be avoidedusing a centring device 34 made of material having low thermalconductivity. Such centring devices are located at a minimum ofpredetermined points along the longitudinal axis and are attached to theoutside of the inner tubing string so as to avoid any metallic contactand to minimise contact between the inner and outer tubular strings withthe exception of the extreme ends of the thermally insulated dual tubingstring.

If both tubing strings are actually locked against each other at theirextreme opposite ends, as shown in FIG. 4, temperature fluctuations maylead to undesirably high axial stresses in one or other tubing strings.FIGS. 7(a) and 7(b) show an embodiment of the invention in which anaxial length compensator 35 is located in a tube of the inner tubingstring 21, although it is to be understood that the compensator may alsobe used in a tube of the outer tubing string 22, or in both strings.

Referring now to FIG. 8, a double walled insulated tubing string 46 ofthis invention is installed inside a production casing string 47 of acemented borehole. Typically, the borehole is provided with a firstcasing string 48, a radially inner second casing string 49 and yetanother, third radially inner casing string 50. Both inner and outertubing strings of the thermally insulated dual tubing string aresupported by the axial motion preventer 25 formed by the well head 58and, thus, both inner and outer strings 21, 22 are fixed againstrelative axial movement. In close proximity to the axial motionpreventer 25 is the seal 26 closing the annular space between bothtubing strings. At the lower end on the thermally insulated tubingstring 46 are the seals 27, 28 (not shown in FIG. 8) and the lockingmember 32 (not separately shown in FIG. 8) which may be provided.

In this embodiment of the invention, a vacuum pump may be connected tothe closed annular gap via valve 29 between the two tubing strings tocreate a vacuum acting as the insulating medium inside the thermallyinsulated double walled tubing string 46. When withdrawing geothermalenergy from the formations surrounding the well, a cold fluid mediumsuch as a liquid, preferably water, is pumped via a valve 51 down thespace between the production casing string 47 and the double walledinsulating tubing string 46. The temperature of rock formationsincreases with depth so that the circulating medium becomes warmer as itapproaches the lower end of the tubing string 46. The circulating liquidthen flows back to the surface through the tubes of the inner tubingstring 21 and is removed via valve 52. Preferably, the cross-sectionalarea of the inner tubing string of the double walled insulated tubingstring 46 is much smaller than the flow area between the double walledinsulating tubing string and the production casing string 47 so that theperiod during which the circulating liquid might lose thermal energywhile it flows back to the surface is less than the time available toheat the medium up when it moves downwardly through the productioncasing string 47.

FIG. 9 shows another embodiment of the present invention using thedouble walled insulated tubing string for an oil producing well. Indistinction to the embodiment of FIG. 8, no cold circulating liquid ispumped down the gap between the insulated tubing string 46 and theproduction casing string 47 so that the temperature difference betweenthe liquid flowing upwards through the insulated tubing string 46 andthe surrounding environment is less severe. The rate of flow of oilproducing wells is, however, frequently extremely small so that theoverall temperature losses are still considerable if the tubing stringis not insulated. Oil, especially with a high content of bitumen orparaffin is extremely temperature sensitive. Once the temperature of theoil or the internal wall of a tubing string falls below a minimumspecific for the particular type of oil, bitumen or paraffin partiallysolidifies so as to block the flow path inside the tubing string oreventually cause the breakage of pump sucker rods. The objective of thethermally insulated dual tubing string of this invention is, therefore,to maintain the temperature of the oil all the way up to the surfacewell above the critical temperature for solidification. In theembodiment of FIG. 9 the production casing string 47 is shown passingthrough an oil-containing formation or aquifer 60.

FIG. 10 shows an enlarged view of the well head used in the embodimentsof FIGS. 8 and 9.

One embodiment of a lower sealing means for the annular space betweeninner and outer tubing strings, and to plug the outer tubing string isshown in FIG. 11. A polished stinger 70 is threadably connected to thelower end of the inner tubing string 21 to seal against seal elements27(a) and 27(b) of seal 27, referred to in FIGS. 3 and 4. A bottom sub71 is connected to the lower end of the outer tubing string 22 and theseals 27(a), 27(b) form a liquid and gas-tight seal between the bottomsub 71 and the stinger 70. The bottom sub 71 is formed in two parts71(a), with which the seals 27(a), 27(b) cooperate, and a lower part71(b) with which seals 28(a), 28(b) of the seal 28, shown in FIGS. 3 and4, cooperate. The plug 31 is located in the lower part 71(b) of thebottom sub. The plug 21 is inserted and sealed against the bottom subpart 71(b) to prevent ingress of liquid into the dual tubing string 46while the dual tubing string is run into the well. As mentioned above,if found desirable for a particular application, both inner and outerstrings may be axially fixed to one another near the seal 28. A deviceknown in the oil industry as a "snap latch" is one manner of effectingsuch axial fixture.

The method of installing a double walled insulating tubing string willnow be described.

Starting with the bottom sub 71 preferably having permanent seals 27(a),27(b), 28(a), 28(b) and temporary plug 31 installed, the outer tubingstring 22 is run into the well. The temporary plug 31 prevents liquidscontained in the well from entering the inside of the string. The bottomsub is connected by a leak-proof connection to a first outer tube of theouter tubing string and as many other tubes are connected as isnecessary until the bottom sub reaches its final position, whereupon theouter tubing string is hung up in slips at the well head 58 (shown inFIGS. 8 and 9).

Commencing with the polished stinger 20, the inner tubing string is nowrun into the empty outer tubing string until the polished stinger 20reaches the bottom sub 71 and seals thereagainst by virtue of seals27(a), 27(b). To equalise the hydrostatic pressure against the bottom ofthe temporary plug 31 caused by the liquid in the well bore, the innertubing string 21 is filled with liquid and the temporary plug 31 isthereafter deactivated, for example by being pumped open. Before orafter filling the inner string with liquid and deactivating thetemporary plug 31, the inner tubing string is hung up in the well headand sealed off against the outer string at the top of the well. To checkif the gap (annular space) between both tubing strings is hermeticallysealed, the gap may not be initially evacuated but closed off while apressure gauge is control the development of the pressure in the gapbetween both inner and outer strings. If the pressure continues to riseafter a small initial increase caused by the initial temperatureincrease of air between the strings, this is an indication of one ormore leaking connections, one or more holes in the tubes and/or aleaking seal at the bottom of the string. In such a case the insulationhas to be immediately rectified. As soon as the system is sealed, avacuum pump is attached to the valve 29 between both tubing strings andthe gap is evacuated. Following this procedure the well may be used toproduce water or oil or to produce thermal energy as a closed loopgeothermal system.

In some circumstances it may be desirable to pretension the dual wallinsulated tubing string by pulling on the inner string against the outerstring to thereby stretch the inner tubing string and compress the outertubing string. Such pre-tensioning may be required in severe thermalconditions.

I claim:
 1. A double walled insulated tubing string adapted to be hungfrom support means, said tubing string comprising inner and outer tubeswith a thermally insulating gap between said inner and outer tubes, saidtubing string comprising plural lengths of outer tubes mechanicallyconnected together by force transferring means to form an outer tubingstring and plural lengths of inner tubes mechanically joined together byfurther force transferring means to form an inner tubing string, saidinner and outer tubing strings being discrete and separate from oneanother over substantially the whole lengths thereof so that saidinsulating gap is continuous from an upper end of the tubing string to abottom end of the tubing string, and seal means located at a bottom endof the tubing string sealing between said inner and outer tubing stringsthereby closing said insulating gap at said bottom end of the tubingstring.
 2. A double walled tubing string as claimed in claim 1 wherein aremovable sealing means is provided at a lower end of the outer tubingstring to prevent liquid ingress into the inner tubing string.
 3. Adouble walled tubing string as claimed in claim 1 wherein the insulatinggap is filled with a gas or liquid medium or said gap is substantiallyevacuated.
 4. A double walled tubing string as claimed in claim 1wherein the inner and outer tubing strings are connected together atboth extreme ends of the tubing string, that is a top end and the bottomend of the insulated tubing string only.
 5. A double walled tubingstring as claimed in claim 1 wherein the inner and outer tubes areadapted to withstand a pressure exceeding a hydro-static head of fluidin or outside the insulated tubing string.
 6. A double walled tubingstring as claimed in claim 1 wherein the inner tubing string ispre-tensioned and the outer tubing string is pre-compressed so as not toexceed predetermined permissible stress levels in the inner and outertubing strings respectively caused by internal and external temperaturevariations.
 7. A double walled tubing string as claimed in claim 6wherein at least one of the inner tubing string and the outer tubingstring is provided with axial length temperature compensating means. 8.A double walled tubing string as claimed in claim 1 wherein aninsulating spacing means is provided in said insulating gap formaintaining concentricity between the inner and outer tubing strings soas to avoid the inner tubing string contacting the outer tubing string.9. A method of installing a double walled insulating tubing stringincluding the steps of:providing a first length of outer tube withsealing means at a lower, in use, end thereof for preventing liquidingress into said outer tube, mechanically connecting a second length ofouter tube to an end of said first outer length remote from said sealingmeans to form an outer tubing string, suspending said outer tubingstring from support means, locating first and second connected lengthsof inner tube forming an inner tubing string inside said outer tubingstring, said first and second inner lengths of tube being mechanicallyconnected together, and said inner tubing string being spaced from saidouter tubing string to provide a thermal insulation gap between theinner and outer tubing strings, said inner and outer tubing stringsbeing discrete and separate from one another over substantially thewhole lengths thereof.
 10. A method as claimed in claim 9 wherein morethan two lengths of outer tube and inner tube are connected,respectively, together.
 11. A method as claimed in claim 9 wherein afluid pressure within the inner tube lengths is increased to remove saidsealing means or said sealing means is removed by mechanical means. 12.A method as claimed in claim 11 wherein a hydro-static pressure againstthe outside of said sealing means in a well bore is equalised by fillingthe internal tube string with a liquid and said sealing means issubsequently de-activated.
 13. A method as claimed in claim 9 whereinthe inner and outer tubes are mechanically connected together at anupper end of the double walled insulating tubing string or at theextreme ends of the double walled insulating tubing string.
 14. A methodas claimed in claim 9 wherein a connection between respective outertubes and between respective inner tubes is a leak-proof connection. 15.A method as claimed in claim 9 wherein at a lower end of the doublewalled insulated tubing string a bottom sub is connected on the outertubing string and a stinger is connected on the inner tubing string soas to form a seal with the outer tubing string.
 16. A method as claimedin claim 9 wherein a seal is provided for the gap at the top of thestring so as to provide a vacuum in said gap.
 17. A method as claimed inclaim 9 wherein where the double walled insulating tubing string islocated in a production casing string, the cross-sectional area betweensaid double walled insulating tubing string and the production casingstring is greater than the cross-sectional area of the inside of theinner tube.
 18. A method as claimed in claim 9 wherein the temperatureat the gap at the lower end of the string is approximately equal to thetemperature at the lower end of the inner tubing string when in use. 19.A method as claimed in claim 9 wherein axial length temperaturecompensating means are included in either the inner and/or outer tubingstrings.
 20. A method of installing a double walled insulating tubingstring including the steps of:providing a first length of outer tubewith sealing means (31) at a lower, in use, end thereof for preventingliquid ingress into said outer tube, mechanically connecting a secondlength of outer tube to the end of said first outer length remote fromsaid sealing means (31) to form an outer tubing string (22), suspendingsaid outer tubing string (22) from support means (58), locating at leastone length of inner tube (21) forming an inner tubing string (21) insidesaid outer tubing string (22), suspending said inner tubing string fromsupport means (58), and said inner tubing string (21) being spaced fromsaid outer tubing string (22) to provide a thermal insulation gap (30)between the inner and outer tubing strings (21, 22), said inner andouter tubing strings being discrete and separate from one another oversubstantially the whole lengths thereof.
 21. A double walled insulatedtubing string adapted to be hung from a support means, said tubingstring comprising inner and outer tubes with a thermally insulating gapbetween said inner and outer tubes, said tubing string comprising plurallengths of outer tubes mechanically connected together to form an outertubing string and plural lengths of inner tubes mechanically joinedtogether to form an inner tubing string, said inner and outer tubingstrings being discrete and separate from one another over substantiallythe whole lengths thereof, and a removable sealing means provided at alower end of the outer tubing string to prevent liquid ingress into theinner tubing string.